1. Field of the Invention
The present invention relates to a magnetooptical recording reproducing apparatus, and a method for determining the power of a light beam in the apparatus.
2. Related Background Art
There have been active developments in magnetooptical record/reproducing apparatus as a memory of low cost and large capacity adapted for use in computers, image filing systems or the like. Such magnetooptical record/reproducing apparatus can be roughly classified into so-called light modulation systems and so-called magnetic field modulation systems. In the apparatus of the light modulation system, the magnetization of a magnetic layer of a recording medium is aligned in advance in a given direction, and the magnetic layer is irradiated with a light beam modulated according to information while a magnetic field is applied at the same time in a direction opposite to the above mentioned direction. The magnetic layer is heated, in a portion thereof irradiated with the light beam, close to the Curie temperature of the magnetic layer, and, in the course of cooling thereof, the irradiated portion alone is magnetized in the direction of the applied magnetic field, whereby the information is recorded as an array of pits in which the direction of magnetization is different from that of the remaining area.
On the other hand, in the apparatus of the magnetic field modulation system, a magnetic field modulated according to information is applied to a magnetooptical recording medium, which is scanned with a light beam of a constant power. The magnetic layer of the medium is locally heated, by the scanning with the light beam, close to the Curie temperature thereof. The heated portion, in the course of cooling, is magnetized in the direction of the applied magnetic field. Thus, as in the light modulation system, the information is recorded as an array of pits in which the direction of magnetization is different from that of the remaining area.
The information thus recorded is read by scanning the recording medium with a linearly polarized light beam of a low power (not heating the magnetic layer close to the Curie temperature thereof), and detecting the reflected light of the light beam by a photodetector through an analyzer. The principle of information reading is based on a so-called magnetooptical effect that the polarized state of the reflected light varies depending on the direction of magnetization of the magnetic layer.
As either of the above-mentioned methods utilizes the heating of the recording medium by the light beam, it is very important to determine and control the power of the light beam. For this reason, the conventional magnetooptical record/reproducing apparatus is equipped with a control circuit for controlling the power of a semiconductor laser employed as the light beam source.
FIG. 1 is a schematic block diagram of a laser power control circuit for the semiconductor laser, employed in the conventional magnetooptical information record/reproducing apparatus, wherein shown are a semiconductor laser unit 32 integrally containing a semiconductor laser 32a for emitting a light beam, and a photosensor 32b provided in the vicinity thereof, for detecting the light amount of the laser beam; a current-voltage (I-V) conversion circuit 33 for converting the photocurrent of the photosensor 32b into a voltage signal; a temperature sensor 34 provided in the vicinity of an optical head; and a controller 35 for calculating the laser power from the output of the I-V converter 33, the output of the temperature sensor 34, revolution of the disk obtained as system information, and its radial position, thereby controlling an LD driver 36.
For setting the recording laser power in the above-explained laser power control circuit, at first the semiconductor laser 32a is driven with a predetermined first drive current designated by the controller 35 in a recording laser power range (APC range). In this state the light amount of the semiconductor laser 32a is monitored by the photosensor 32b, and is converted by the I-V converter into a voltage signal. The controller 35 calculates the output laser power, utilizing the predetermined relationship between the laser power output and the monitor output of the photosensor 32b. Then the controller 35 instructs the LD driver 36 to drive the semiconductor laser 32a with a predetermined second drive current, and calculates the output laser power, based on the monitor output of the photosensor 32b and the pre-entered value. The controller 35 determines the correlation between the drive current of the semiconductor laser 32a and the laser power from a thus obtained relationship between the output laser power and two driving currents, and determines the drive current for obtaining a desired laser power from such a correlation, thereby controlling the LD driver 36. The desired laser power means the optimum laser power for the magnetooptical recording medium, in consideration of the revolution of the magnetooptical disk, the linear speed determined by the radial position, and the atmospheric temperature in the apparatus.
More specifically, FIG. 2 shows the relationship, at the information recording on the magnetooptical disk, between the laser power and the carrier-to-noise (C/N) ratio or the carrier-to-noise (C.sub.2 /N.sub.2) ratio of the secondary harmonic wave. The employed recording conditions are: a magnetooptical disk of 3.5 inches, a revolution of 3000 rpm, a recording frequency of 4.8 MHz, a recording position of 24 mm in the radius of the disk, and recording at room temperature. As shown in FIG. 2, the C/N ratio does not show a large change when the laser power is 4 mW or larger, but the C.sub.2 /N.sub.2 ratio for the secondary harmonic wave assumes a minimum value at a laser power of about 4.5 mW and becomes larger at a lower or higher laser power. In general, a laser power at which the secondary harmonic wave is minimized is considered to be an optimum recording power according to which recording of information can be effected with least error. It is also known that the optimum laser power is dependent on the temperature of the magnetooptical disk, and is linearly correlated therewith.
For these reasons, the laser power has to be varied according to the temperature of the magnetooptical disk in order to achieve optimum recording, and for this purpose the laser power has to be controlled by detecting the temperature of the disk. It has therefore been customary to provide a temperature sensor in the magnetooptical information record/reproducing apparatus or to directly mount a temperature sensor on the disk cartridge and to control the laser power according to the result of temperature detection.
However, in such prior art, the exact control of the laser power in an optimum manner has been difficult, because the temperature detection is conducted in the interior of the apparatus or on the disk cartridge, instead of the magnetooptical disk itself. In more detail, when the temperature sensor is provided inside the apparatus, it merely detects the temperature of the atmosphere in said apparatus, and the temperature fluctuates by the direction or amount of the air flow within the apparatus, or by the location within the apparatus. For this reason it is not possible to exactly detect the temperature of the magnetooptical disk. Also, since the magnetooptical disk is detached from and mounted in the apparatus, there will result a significant temperature difference between the temperature detected by the temperature .sensor and the actual temperature of the magnetooptical disk, when it is mounted in the apparatus of which internal temperature is already elevated. In such a case, for a certain time after the mounting of the disk, the recording is conducted with a laser power distant from the optimum power, thus leading to recording errors. Also, in a case of detecting the temperature of the disk cartridge, it is still difficult to exactly detect the disk temperature, because there is still a temperature difference between the cartridge surface and the magnetooptical disk due to the differences in the thermal capacity or thermal conductivity between the disk and cartridge. For this reason the exact control of the laser power at the optimum power level has been difficult.
Also, in the conventional magnetooptical record/reproducing apparatus, it has been difficult to exactly set the recording laser power at the optimum value, because of fluctuations in sensitivity among the recording media or because of the limitations in the accuracy of atmospheric temperature measurement due to the precision of the thermistor employed as the temperature sensor. Furthermore, because of the temperature characteristics of the photosensor employed for monitoring the light amount of the semiconductor laser and the loss of laser power on the magnetooptical recording medium due to stains on the lenses in the optical system, the setting accuracy of the output laser power becomes deteriorated in the conventional control device, so that it has been difficult to control the laser power exactly at the desired power level.